The decision to remove

The primary concern with shadow-piercing selectors is that they violate
encapsulation and create situations where a component can no longer change its
internal implementation.

Note: For the moment, ::shadow and /deep/ will continue to work with
JavaScript APIs like querySelector() and querySelectorAll(). Ongoing support
for these APIs is being discussed on
GitHub.

The CSS Shadow Parts spec
is being advanced as an alternative to shadow piercing selectors. Shadow Parts
will allow a component author to expose named elements in a way that preserves
encapsulation and still allows page authors the ability to style multiple
properties at once.

What should I do if my site uses ::shadow and /deep/?

The ::shadow and /deep/ selectors only affect legacy Shadow DOM v0
components. If you're using Shadow DOM v1, you should not need to change
anything on your site.

You can use Chrome Canary
to verify your site does not break with these new changes. If you notice issues,
try and remove any usage of ::shadow and /deep/. If it's too difficult to
remove usage of these selectors, consider switching from native shadow DOM over
to the shady DOM polyfill. You should only need to make this change if your site
relies on native shadow DOM v0.